U.S. patent application number 12/540529 was filed with the patent office on 2009-12-10 for electrolyte for lithium ion secondary battery and lithium ion secondary battery comprising the same.
This patent application is currently assigned to Samsung SDI Co., Ltd.. Invention is credited to Suhee Han, Jinbum Kim, Jinsung Kim, Jinhyunk Lim, Jungkang Oh, Narae Park.
Application Number | 20090305145 12/540529 |
Document ID | / |
Family ID | 41076940 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090305145 |
Kind Code |
A1 |
Kim; Jinsung ; et
al. |
December 10, 2009 |
ELECTROLYTE FOR LITHIUM ION SECONDARY BATTERY AND LITHIUM ION
SECONDARY BATTERY COMPRISING THE SAME
Abstract
An electrolyte for a lithium ion secondary battery includes a
non-aqueous organic solvent; lithium salt; and difluoro oxalato
borate and fluoro ethylene carbonate (FEC). The capacity retention
property and durability of a lithium ion secondary battery
including the electrolyte is excellent even when the battery is
left at a high temperature.
Inventors: |
Kim; Jinsung; (Yogin-si,
KR) ; Park; Narae; (Yongin-si, KR) ; Lim;
Jinhyunk; (Yongin-si, KR) ; Han; Suhee;
(Yongin-si, KR) ; Kim; Jinbum; (Yongin-si, KR)
; Oh; Jungkang; (Yongin-si, KR) |
Correspondence
Address: |
STEIN MCEWEN, LLP
1400 EYE STREET, NW, SUITE 300
WASHINGTON
DC
20005
US
|
Assignee: |
Samsung SDI Co., Ltd.
Suwon-si
KR
|
Family ID: |
41076940 |
Appl. No.: |
12/540529 |
Filed: |
August 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12256781 |
Oct 23, 2008 |
7592102 |
|
|
12540529 |
|
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Current U.S.
Class: |
429/337 ;
29/623.1; 429/199; 429/207; 429/341; 429/342; 429/343 |
Current CPC
Class: |
H01G 9/022 20130101;
Y10T 29/49108 20150115; Y02E 60/10 20130101; H01M 10/0567 20130101;
H01M 10/4235 20130101; H01M 10/0525 20130101 |
Class at
Publication: |
429/337 ;
429/207; 429/341; 429/342; 429/343; 429/199; 29/623.1 |
International
Class: |
H01M 6/16 20060101
H01M006/16; H01M 10/26 20060101 H01M010/26; H01M 10/36 20060101
H01M010/36 |
Foreign Application Data
Date |
Code |
Application Number |
May 21, 2008 |
KR |
10-2008-47027 |
Claims
1. An electrolyte for a lithium ion secondary battery comprising: a
non-aqueous organic solvent; a lithium salt; and difluoro oxalato
borate (DFOB) represented by a chemical formula 1 below and fluoro
ethylene carbonate (FEC) as additives, ##STR00004##
2. The electrolyte of claim 1, wherein the non-aqueous organic
solvent is at least one selected from the group consisting of a
carbonate, an ester, an ether and a ketone.
3. The electrolyte of claim 2, wherein the carbonate is at least
one selected from the group consisting of dimethyl carbonate,
diethyl carbonate, dipropyl carbonate, methylpropyl carbonate,
ethylpropyl carbonate, ethylmethyl carbonate, ethylene carbonate,
propylene carbonate, butylene carbonate and pentylene
carbonate.
4. The electrolyte of claim 2, wherein the ester is at least one
selected from the group consisting of n-methyl acetate, n-ethyl
acetate, n-propyl acetate, dimethyl acetate, methyl propionate,
ethyl propionate, .gamma.-butyrolactone, decanolide, valerolactone,
mevalonolactone and caprolactone.
5. The electrolyte of claim 2, wherein the ether is at least one
selected from the group consisting of dibutyl ether, tetraglyme,
diglyme, dimethoxy ethane, 2-methyltetrahydrofuran and
tetrahydrofuran.
6. The electrolyte of claim 2, wherein the ketone is at least one
selected from the group consisting of cyclohexanone and
polymethylvinyl ketone.
7. The electrolyte of claim 1, wherein the lithium salt is at least
one selected from the group consisting of LiPF.sub.6, LiBF.sub.4,
LiSbF.sub.6, LiAsF.sub.6, LiClO.sub.4, LiCF.sub.3SO.sub.3,
LiC.sub.4F.sub.9SO.sub.3, LiAlO.sub.4, LiAlCl.sub.4,
LiN(C.sub.pF.sub.2p+1 SO.sub.2)(C.sub.qF.sub.2q+1 SO.sub.2) (where
p and q are natural numbers), LiCl and Lil.
8. A lithium ion secondary battery, comprising: a cathode including
a cathode active material that can reversibly intercalate and
deintercalate lithium; an anode including an anode active material
that can reversibly intercalate and deintercalate lithium; and the
electrolyte of claim 1.
9. A lithium ion secondary battery, comprising: a cathode including
a cathode active material that can reversibly intercalate and
deintercalate lithium; an anode including an anode active material
that can reversibly intercalate and deintercalate lithium; and the
electrolyte of claim 2.
10. A lithium ion secondary battery, comprising: a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium;
and the electrolyte of claim 3.
11. A lithium ion secondary battery, comprising: a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium;
and the electrolyte of claim 4.
12. A lithium ion secondary battery, comprising: a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium;
and the electrolyte of claim 5.
13. A lithium ion secondary battery, comprising: a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium;
and the electrolyte of claim 6.
14. A lithium ion secondary battery, comprising: a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium;
and the electrolyte of claim 7.
15. A method of improving a capacity retention property, thermal
stability and durability of a lithium ion secondary battery, the
method comprising: injecting an electrolyte comprising a
non-aqueous organic solvent; a lithium salt; difluoro oxalato
borate and fluoro ethylene carbonate (FEC) into a battery can
including an electrode assembly comprising a cathode including a
cathode active material that can reversibly intercalate and
deintercalate lithium; an anode including an anode active material
that can reversibly intercalate and deintercalate lithium to form a
lithium ion secondary battery; and charging the lithium ion
secondary battery or exposing the lithium ion secondary battery to
a temperature of 60.degree. C. or greater.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
12/256,781 filed on Oct. 23, 2008, now pending, which claims the
benefit Korean Application No. 10-2008-0047027, filed on May 21,
2008 in the Korean Intellectual Property Office, the disclosures of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Aspects of the present invention relate to an electrolyte
for a lithium ion secondary battery and a lithium ion secondary
battery comprising the same. More particularly, aspects of the
present invention relate to an electrolyte that can improve
capacity retention property, thermal stability and durability of a
lithium ion secondary battery, even when the battery is left at a
high temperature, by containing difluoro oxalato borate (DFOB) and
fluoro ethylene carbonate (FEC) as additives, and a lithium ion
secondary battery comprising the same.
[0004] 2. Description of the Related Art
[0005] A battery is a device that converts the chemical energy of
chemical materials inside the battery into electrical energy
through an electrochemical oxidation/reduction reaction. Recently,
portable devices such as camcorders, cellular phones, notebook
computers, PCs and PDAs have been actively developed with rapid
progress of the electronic, telecommunication and computer
industries. Accordingly, there has been an increased demand for a
slim secondary battery of high performance, durability and
reliability that can be used in the above portable devices.
[0006] A lithium ion battery has been widely used as a secondary
battery because it has a high discharge voltage near 4V, an
excellent energy density per weight and a low self-discharge
rate.
[0007] In the lithium secondary battery, the ion conductivity of
the electrolyte greatly affects the charge/discharge performance of
the battery. Thus, it is desirable that an electrolyte have a high
ion conductivity. Accordingly, in the battery industry, many
experiments have been performed to improve electrochemical
characteristics such as the ion conductivity of the battery by
mixing a solvent of a high dielectric constant and a solvent of a
low viscosity. In addition, research has been actively conducted to
improve the thermal stability of the battery by mixing a solvent of
a high boiling point (Japanese Patent publication No.
1999-111306).
[0008] During initial charging of the lithium battery, lithium ions
released from a positive electrode move to a carbon electrode used
as a negative electrode and form a film on the surface of the
negative electrode. Such a film is called as a SEI (solid
electrolyte interface) film. The SEI film greatly affects on
discharge capacity during subsequent cycles. Physical and chemical
properties of the SEI film are changed according to salt used in
the electrolyte, the concentration of the salt, components of
solvent mixture, composition of the solvent mixture and the kind of
additive used. When an uneven SEI film is formed, because an
additive is not used or because undesirable additive is used,
electrons in active materials are released and cause decomposition
of the electrolyte. Accordingly, the irreversible capacity of the
active material is increased and the capacity and lifetime of the
battery is reduced. With respect to the SEI film, there has been
much research directed to improving the physical and chemical
properties of the film by changing the SEI film formation reaction
by adding an additive to the electrolyte. For example, there have
been disclosed techniques of adding CO.sub.2 to the electrolyte
(Japanese Patent publication No. 1995-176323A) or preventing
decomposition of the electrolyte by adding a sulfide group compound
to the electrolyte (Japanese Patent publication No.
1995-320779A).
[0009] Accordingly, an electrolyte having high ion conductivity,
high dielectric constant and low viscosity is desirable in the
lithium secondary battery. In addition, a solvent capable of
reducing decomposition and vaporization of the electrolyte after
discharge is desirable to improve discharge characteristics of the
battery at a high temperature. In addition, an additive is
desirable to form an even SEI film that transfers lithium ions well
but does not transfer other material.
SUMMARY OF THE INVENTION
[0010] Accordingly, aspects of the present invention provide an
electrolyte for a lithium ion secondary battery that can improve
capacity retention property, thermal stability and durability even
when the battery is left at a high temperature.
[0011] Additional advantages, aspects and features of the invention
will be set forth in part in the description which follows and in
part will become apparent to those having ordinary skill in the art
upon examination of the following or may be learned from practice
of the invention.
[0012] According to an embodiment of the present invention, there
is provided an electrolyte for a lithium ion secondary battery that
includes a non-aqueous organic solvent; lithium salt; and difluoro
oxalato borate (DFOB) represented by a chemical formula 1 below and
fluoro ethylene carbonate (FEC) as additives, where the amount of
difluoro oxalato borate is 0.1 to 5 parts by weight based on 100
parts by weight of the total electrolyte and the amount of the
fluoro ethylene carbonate is 0.1 to 10 parts by weight based on 100
parts by weight of the total electrolyte:
##STR00001##
[0013] According to an aspect of the present invention, the amount
of the difluoro oxalato borate represented by the chemical formula
1 may be 0.5 to 5 parts by weight based on 100 parts by weight of
the total electrolyte and the amount of the fluoro ethylene
carbonate may be 2 to 10 parts by weight based on 100 parts by
weight of the total electrolyte.
[0014] According to an aspect of the present invention, the
non-aqueous organic solvent may be at least one selected from the
group consisting of a carbonate, an ester, an ether and a
ketone.
[0015] According to an aspect of the present invention, the
carbonate may be at least one selected from the group consisting of
dimethyl carbonate, diethyl carbonate, dipropyl carbonate,
methylpropyl carbonate, ethylpropyl carbonate, ethylmethyl
carbonate, ethylene carbonate, propylene carbonate, butylene
carbonate and pentylene carbonate.
[0016] According to an aspect of the present invention, the ester
may be at least one selected from the group consisting of methyl
acetate, ethyl acetate, propyl acetate, dimethyl acetate, methyl
propionate, ethyl propionate, .gamma.-butyrolactone, decanolide,
valerolactone, mevalonolactone and caprolactone.
[0017] According to an aspect of the present invention, the ether
may be at least one selected from the group consisting of dibutyl
ether, tetraglyme, diglyme, dimethoxy ethane,
2-methyltetrahydrofuran and tetrahydrofuran.
[0018] According to an aspect of the present invention, the ketone
may be at least one selected from the group consisting of
polymethylvinyl ketone and cyclohexanone.
[0019] According to an aspect of the present invention, the lithium
salt may be at least one selected from the group consisting of
LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiClO.sub.4,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3, LiAlO.sub.4,
LiAlCl.sub.4, LiN(C.sub.pF.sub.2p+1 SO.sub.2)(C.sub.qF.sub.2q+1
SO.sub.2) (where p and q are natural numbers), LiCl and Lil.
[0020] According to another embodiment of the present invention,
there is provided a lithium ion secondary battery, which comprises:
a cathode including a cathode active material that can reversibly
intercalate and deintercalate lithium; an anode including an anode
active material that can reversibly intercalate and deintercalate
lithium; and the above electrolyte according to aspects of the
present invention.
[0021] According to another embodiment of the present invention,
there is provided a method of improving a capacity retention
property, thermal stability and durability of a lithium ion
secondary battery, the method comprising injecting an electrolyte
comprising a non-aqueous organic solvent; a lithium salt; difluoro
oxalato borate and fluoro ethylene carbonate (FEC), wherein the
amount of the difluoro oxalato borate is 0.1 to 5 parts by weight
based on 100 parts by weight of the total electrolyte and the
amount of the fluoro ethylene carbonate is 0.1 to 10 parts by
weight based on 100 parts by weight of the total electrolyte, into
a battery can including an electrode assembly comprising a cathode
including a cathode active material that can reversibly intercalate
and deintercalate lithium; an anode including an anode active
material that can reversibly intercalate and deintercalate lithium
to form a lithium ion secondary battery; and charging the lithium
ion secondary battery or exposing the lithium ion secondary battery
to a temperature of 60.degree. C. or greater.
[0022] Additional aspects and/or advantages of the invention will
be set forth in part in the description which follows and, in part,
will be obvious from the description, or may be learned by practice
of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] These and/or other aspects and advantages of the invention
will become apparent and more readily appreciated from the
following description of the embodiments, taken in conjunction with
the accompanying drawings of which:
[0024] FIG. 1 is a view illustrating a rectangular type lithium ion
secondary battery according to one exemplary embodiment of the
present invention;
[0025] FIG. 2 is a graph illustrating capacity relative to the
number of charge/discharge cycle according to Embodiments 1 to 5
and Comparison Examples 1 to 3; and
[0026] FIG. 3 is a graph illustrating capacity retention ratio
relative to the number of charge/discharge cycle according to
Embodiments 1 to 5 and Comparison Examples 1 to 3.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0027] Reference will now be made in detail to the present
embodiments of the present invention, examples of which are
illustrated in the accompanying drawings, wherein like reference
numerals refer to the like elements throughout. The embodiments are
described below in order to explain the present invention by
referring to the figures.
[0028] An electrolyte according to aspects of the present invention
includes difluoro oxalato borate (DFOB), represented by a chemical
formula 1 below, and fluoro ethylene carbonate (FEC) as
additives:
##STR00002##
[0029] The difluoro oxalato borate is added to the electrolyte with
fluoro ethylene carbonate. The difluoro oxalato borate prevents
loss of lithium caused by reaction between lithium and electrolyte
by forming a film having high thermal stability on an anode at the
time of initial charging or forms the film on an electrode active
material when the battery is exposed to a high temperature. In
addition, the difluoro oxalato borate has a high
oxidation-decomposition voltage and thus can prevent degradation of
cycle characteristics of the battery and can improve thermal
stability by preventing decomposition of the electrolyte. According
to one embodiment, the amount of the difluoro oxalato borate may be
0.1 to 5, or more specifically, 0.5 to 5 parts by weight to 100
parts by weight of the total electrolyte, and the amount of the
fluoro ethylene carbonate may be 0.1 to 10, or more specifically, 2
to 10 parts by weight to 100 parts by weight of the total
electrolyte. When the amount of the additive is less than the above
range, improvement of the thermal stability and durability may be
negligible. On the other hand, when the amount of the additive is
more than the above range, the viscosity of the electrolyte is
increased, which may reduce the movement of lithium ions and
adversely affect the thermal stability.
[0030] The electrolyte also includes a non-aqueous organic solvent
and lithium salts. The non-aqueous organic solvent functions as a
medium that transfers lithium ions participated in the
electrochemical reaction of the battery. The non-aqueous organic
solvent may be at least one selected from the group consisting of a
carbonate, an ester, an ether and a ketone, or a mixture
thereof.
[0031] The carbonate group solvent may be dimethyl carbonate (DMC),
diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl
carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate
(EMC), ethylene carbonate (EC), propylene carbonate (PC) or
butylene carbonate (BC).
[0032] The ester group solvent may be n-methyl acetate, n-ethyl
acetate, n-propyl acetate, dimethyl acetate, methyl propionate,
ethyl propionate, .gamma.-butyrolactone, decanolide, valerolactone,
mevalonolactone or caprolactone.
[0033] The ether may be dibutyl ether, tetraglyme, diglyme,
dimethoxy ethane, 2-methyltetrahydrofuran or tetrahydrofuran.
[0034] The ketone may be cyclohexanone or polymethylvinyl
ketone.
[0035] The non-aqueous organic solvent may be used alone, or a
mixture of solvents may be used. When a mixture of organic solvents
is used, the mixing ratio may be properly controlled according to
the desired battery performance. An organic solvent having a high
dielectric constant and a low viscosity may be used to transfer
ions smoothly by improving the dissociation of ions. Generally, it
is desirable to use a mixture of at least two solvents including a
solvent having high dielectric constant and viscosity and a solvent
having low dielectric constant and viscosity. It is desirable to
use a mixture of a cyclic type carbonate and a chain type carbonate
as the carbonate group solvent. It is further desirable that the
mixing ratio of the cyclic type carbonate to the chain type
carbonate be 1:1 to 1:9 in volume ratio to improve the performance
of the electrolyte.
[0036] The non-aqueous organic solvent may further include an
aromatic hydrocarbon group organic solvent in addition to the
carbonate group organic solvent. An aromatic hydrocarbon compound
having the following chemical formula 2 may be used as the aromatic
hydrocarbon group organic solvent.
##STR00003##
[0037] where R is halogen or an alkyl group having 1 to 10 carbons
and q is an integer of 0 to 6.
[0038] The aromatic hydrocarbon group organic solvent may be any
one of benzene, fluorobenzene, bromobenzene, chlorobenzene,
toluene, xylene, or mesitylene or mixture thereof. When the volume
ratio of the carbonate solvent to the aromatic hydrocarbon group
organic solvent is 1:1 to 30:1 in the electrolyte containing the
aromatic hydrocarbon group organic solvent, generally desirable
properties of the electrolyte such as stability, safety and ion
conductivity are improved, in comparison to compositions having
other volume ratios.
[0039] The lithium salt functions as a source of lithium ions and
enables basic operations of the lithium ion secondary battery and
promotes movement of lithium ions between the cathode and anode.
The lithium salt may be any one selected from the group consisting
of LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6, LiClO.sub.4,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3, LiAlO.sub.4,
LiAlCl.sub.4, LiN(C.sub.pF.sub.2p+1 SO.sub.2)(C.sub.qF.sub.2q+1
SO.sub.2) (p and q are natural numbers), LiCl and Lil or mixture
thereof. It is desirable to use a lithium salt that has a low
lattice energy, high dissociation degree, excellent ion
conductivity, thermal stability and oxidation resistance. In
addition, it is desirable that the concentration of the lithium
salt be in the range of 0.1 to 2.0M. When the concentration of the
lithium salt is less than 0.1 M, the conductivity of the
electrolyte is decreased and thus, the performance thereof may be
degraded. On the other hand, when the concentration of the lithium
salt is more than 2.0M, the viscosity of the electrolyte is
increased and thus, the movement of the lithium ions may
reduced.
[0040] The lithium secondary battery comprising the electrolyte
includes a cathode, an anode and a separator. The cathode includes
a cathode active material that can reversibly intercalate and
deintercalate lithium ions. As a non-limiting example, the cathode
active material may be a composite metal oxide of lithium and at
least one selected from cobalt, manganese and nickel. The solid
solubility of metals may be variously used in the composite metal
oxide. In addition to these metals, one or more metals selected
from the group consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V,
Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V and rare earth elements may be
further included.
[0041] The anode includes an anode active material that can
reversibly intercalate and deintercalate lithium ions. Crystalline
or amorphous carbon, carbonic anode active material (thermally
decomposed carbon, coke, graphite) such as a carbon composite,
combusted organic polymer, carbon fiber, tin oxide, lithium metal
or an alloy of lithium and another element may be used as the anode
active material. For example, the amorphous carbon may be hard
carbon, coke, mesocarbon microbeads (MCMB) fired below 1500.degree.
C., mesophase pitch-based carbon fiber (MPCF), etc. The crystalline
carbon may be a graphite material, more particularly, natural
graphite, graphitized cokes, graphitized MCMB, or graphitized MPCF,
etc.
[0042] The anode and cathode may be made by preparing an electrode
slurry composition by dispersing the electrode active material, a
binder, a conductive material and a thickener, if desired, in a
solvent and coating the slurry composition onto an electrode
collector. As non-limiting examples, aluminum or an aluminum alloy
may be used as a cathode collector and copper or copper alloy may
be used as an anode collector. The anode and cathode collectors may
be formed as a foil or mesh.
[0043] The lithium secondary battery includes a separator that
prevents a short between the cathode and anode. As non-limiting
examples, a polymer membrane such as a polyolefin, polypropylene or
polyethylene membrane, a multi-membrane thereof, a micro-porous
film, or a woven or non-woven fabric may be used as the
separator.
[0044] A unit battery having a structure of
cathode/separator/anode, a bi-cell having a structure of
cathode/separator/anode/separator/cathode, or a battery stack
including a plurality of unit batteries may be formed using
above-described lithium secondary battery including the
electrolyte, cathode, anode and separator.
[0045] A typical lithium secondary battery having the above
construction is shown in FIG. 1.
[0046] Referring to FIG. 1, the lithium secondary battery is formed
of an electrode assembly 12 including a cathode 13, an anode 15 and
a separator 14, a can 10 receiving the electrode assembly and
electrolyte, and a cap assembly 20 sealing an upper part of the can
10. The cap assembly 20 includes a cap plate 40, an insulation
plate 50, a terminal plate 60 and an electrode terminal 30. The cap
assembly 20 is combined with an insulation case 70 to seal the can
10.
[0047] The electrode terminal 30 is inserted into a terminal hole
41 formed at the middle of the cap plate 40. When the electrode
terminal 30 is inserted into the terminal hole 41, a tubular gasket
46 combined with an outer surface of the electrode terminal 30 is
inserted into the terminal hole 41 to insulate the electrode
terminal 30 from the cap plate 40. After the cap assembly 20 is
fastened onto the upper part of the can 10, the electrolyte is
injected through an electrolyte injection hole 42 and then the
electrolyte injection hole 42 is sealed by a stopper 43. The
electrode terminal 30 is connected to an anode tab 17 of the anode
or a cathode tab 16 of the cathode to function as an anode terminal
or a cathode terminal.
[0048] The lithium secondary battery may be formed in various types
such as cylindrical and pouch types in addition to the rectangular
type.
[0049] Embodiments of the present invention and comparison examples
will be explained below, but the present invention is not limited
thereto.
Embodiment 1
[0050] A cathode active material slurry was prepared by dispersing
LiCoO.sub.2 as a cathode active material, polyvinylidene fluoride
(PVdF) as a binder, and carbon as a conductive material in an
N-methyl-2-pyrrolidone (NM P) solvent at a weight ratio of 92:4:4.
Then, a cathode was formed by coating the cathode active material
slurry onto an aluminum foil having a thickness of 15 .mu.m, and
then drying and rolling the coated foil. The anode active material
slurry was prepared by mixing artificial graphite as an anode
active material, styrene-butadiene rubber as a binder, and
carboxymethylcelluose as a thickener at a weight ratio of 96:2:2,
then dispersing the mixture in water. Then, an anode was formed by
coating the slurry onto a copper foil having a thickness of 10
.mu.m, and drying and rolling the coated foil.
[0051] Next, after a polyethylene (PE) separator having a thickness
of 10 .mu.m was interposed between the above electrodes to form an
electrode assembly, the electrode assembly was wound and
pressurized. Then, the electrode assembly was inserted into a
rectangular can having a size of 55 mm by 34 mm by 50 mm. Then, the
electrolyte described below was injected into the can, thereby
completing a lithium secondary battery.
[0052] The electrolyte was prepared by adding LiPF.sub.6 in a
mixture solvent of ethylene carbonate, ethylmethyl carbonate and
diethyl carbonate (the volume ratio was 1:1:1) and adding difluoro
oxalato borate and fluoro ethylene carbonate, where the
concentration of the LiPF.sub.6 was 1.0M. The amount of difluoro
oxalato borate was 0.5 parts by weight and the amount of the fluoro
ethylene carbonate was 3 parts by weight based on 100 parts by
weight of the total electrolyte.
Embodiment 2
[0053] This embodiment was carried out in the same manner as
Embodiment 1, except that the amount of difluoro oxalato borate was
1 part by weight and the amount of fluoro ethylene carbonate was 3
parts by weight.
Embodiment 3
[0054] This embodiment was carried out in the same manner as
Embodiment 1, except that the amount of difluoro oxalato borate was
1 part by weight and the amount of fluoro ethylene carbonate was 5
parts by weight.
Embodiment 4
[0055] This embodiment was carried out in the same manner as
Embodiment 1, except that the amount of difluoro oxalato borate was
3 parts by weight and the amount of fluoro ethylene carbonate was 5
parts by weight.
Embodiment 5
[0056] This embodiment was carried out in the same manner as
Embodiment 1, except that the amount of difluoro oxalato borate was
0.5 parts by weight and the amount of fluoro ethylene carbonate was
10 parts by weight.
Comparison Example 1
[0057] This comparison example was carried out in the same manneras
Embodiment 1, except that amount of difluoro oxalato borate was 3
parts by weight, and fluoro ethylene carbonate was not added.
Comparison Example 2
[0058] This comparison example was carried out in the same manner
as Embodiment 1, except that difluoro oxalato borate was not added,
and the amount of fluoro ethylene carbonate was 3 parts by
weight.
Comparison Example 3
[0059] This comparison example was carried out in the same manner
as Embodiment 1, except that the amount of difluoro oxalato borate
was 0.5 parts by weight and the amount of fluoro ethylene carbonate
was 15 parts by weight were added.
[0060] The capacity and capacity retention ratios according to the
repetition of charge/discharge cycles were measured for the
batteries manufactured in the embodiments and comparison examples
after the batteries had been left for 30 days at 60.degree. C.
Thus, when the difluoro oxalato borate and fluoro ethylene
carbonate were simultaneously used as additives, a positive effect
on the lifetime and thermal stability of the battery was
confirmed.
Experimental Example 1
Lifetime Test--Capacity and Capacity Retention Ratio According to
Repetition of Charge/Discharge
[0061] The batteries manufactured in the Embodiments 1 to 5 and the
Comparison Examples 1 to 3 were charged with a constant current 1 C
until their voltages reached to 4.2V, after that they were charged
with a constant voltage of 4.2V at a room temperature until total
charging time reached to 3 hrs, and then discharged with a constant
current of 1 C until their voltages reached to 3.0V. Where C is a
unit of `c-rate` and `c-rate` is a charge or discharge current rate
of battery expressed in amperes. The charge/discharge was performed
for 100, 200 and 300 cycles respectively. Then, capacities
corresponding to the cycles were measured, and the results are
shown in FIG. 2. The capacity retention ratios (%) corresponding to
the cycles were calculated as below and the results are shown in
FIG. 3 and Table 1.
Capacity retention ratio (%) of the corresponding
cycle=(discharging capacity of the corresponding cycle/discharging
capacity of the first cycle)*100(%)
Experimental Example 2
Thermal Stability Test--Capacity Retention Property after the
Battery was Left for 30 Days at 60.degree. C.
[0062] The batteries manufactured in Embodiments 1 to 5 and
Comparison Examples 1 to 3 were charged with a constant current and
constant voltage of 0.5 C/4.2V for 3 hours and left for 30 days at
60.degree. C. Then, after the batteries were discharged with 0.5
C/3.0V, the capacity retention ratio was measured. The results are
shown in Table 1.
Capacity retention ratio after the battery was left at a high
temperature=((discharging capacity after the battery was
left-initial capacity)/initial capacity)*100(%).
TABLE-US-00001 TABLE 1 DFOB FEC Capacity retention Capacity
retention ratio(%) (parts by (parts by ratio(%) after 300 after the
battery was left weight) weight) charge/discharge cycles for 30
days at 60.degree. C. Embodiment 1 0.5 3 83 81 Embodiment 2 1 3 88
83 Embodiment 3 1 5 91 80 Embodiment 4 3 5 86 75 Embodiment 5 0.5
10 86 75 Comparison 3 0 74 53 example 1 Comparison 0 3 77 64
example 2 Comparison 0.5 15 86 50 example 3
[0063] As shown in Table 1, when difluoro oxalato borate (DFOB) of
0.1 to 5 parts by weight and fluoro ethylene carbonate (FEC) of 0.1
to 10 parts by weight were added, the capacity retention ratios
after charge/discharge cycles, that is, the lifetime of the
batteries was improved and the capacity retention property was
improved after the battery was left at the high temperature. When
only one of the difluoro oxalato borate or fluoro ethylene
carbonate was added, the capacity retention property was
inferior.
[0064] In addition, referring to FIGS. 2 and 3, in the cases of the
Embodiments 1 to 3 in which the DFOB and FEC were added, the
discharge capacity and capacity retention ratio after
charge/discharge of 300 cycles were excellent compared than the
Comparison Examples.
[0065] As described above, the lithium secondary battery comprising
the electrolyte according to aspects of the present invention
produces the effect that the capacity retention property of the
battery is improved even when the battery is left at a high
temperature. Thus, thermal stability and durability of the battery
are prominently improved.
[0066] Although a few embodiments of the present invention have
been shown and described, it would be appreciated by those skilled
in the art that changes may be made in this embodiment without
departing from the principles and spirit of the invention, the
scope of which is defined in the claims and their equivalents.
* * * * *